Freeholders permission to build extension on a leasehold flat.

Hello everyone.

I would like to build an extension on my ground floor leasehold flat. The extension would be built on common land (i think that's the correct term) the use of which is shared by myself and the upstairs flat. The owners of the other flat are happy for the extension to be build.

My lease says the following

"1) the tenants hereby jointly and severally covenant with the landlord as follows

(c) not to make any structural alterations or structural additions to the demised premises nor to erect any new buildings thereon or to remove the landlords fixtures without the landlords previous consent in writing".

I wrote to the freeholders managing agents and they asked for

1) £500.00 for granting permission

2) £1000 (plus vat) to cover the freeholders legal fees.

On the one hand I'm thinking £1700.00 for permission to build on 14 square metres of land in the South East doesn't seem to bad. On the other hand I'm thinking well the lease doesn't say anything about fees, just the freeholders permission.

I'd be interested to know people opinions on wether the freeholders offer is a good deal.

Thanks as always